This invention relates to electric power steering devices for automotive vehicles including an electric motor for providing a steering torque to assist the driver in directing the road wheels of the vehicle.
FIG. 7 is a diagram showing the overall structure of a typical electric power steering device for an automotive vehicle, as disclosed, for example, in Japanese Kokai Nos. 62-255273 and 63-215461. The driver of the vehicle applies steering torque to a steering wheel 1. A steering shaft 2 transmits the torque from the steering wheel 1. A steering torque sensor 3 provided on the steering shaft 2 detects the steering torque T applied to the steering wheel 1. Universal joints 4 connect the angled parts of the steering shaft 2. A pinion 5 at the bottom end of the steering shaft 2 is engaged with a rack 6 for directing the road wheels of the vehicle. A steering speed sensor 7 detects the steering speed A of the steering wheel 1.
A control unit 9 including a microcomputer generates motor current command level .sub.M and clutch control signal C in response to the vehicle speed V detected by a vehicle speed sensor 10, steering speed A, and steering torque T. The electric power is provided by a battery 11 mounted on the vehicle. A key switch 12 is inserted between the battery 11 and the control unit 9. An electric motor 13 for providing the assisting steering torque is driven by the DC current supplied from the battery 11, in accordance with the motor current command level I.sub.M.
An electromagnetic clutch 14 directly coupled to the output shaft of the electric motor 13 is engaged and disengaged in response to the clutch control signal C. A worm 15 coupled to the output shaft of the electromagnetic clutch 14 is engaged with a worm wheel 16. A pinion 18 coupled to the worm wheel 16 is engaged with the rack 6. Thus, in response to the clutch control signal C, the assisting steering torque provided by the electric motor 13 is selectively transmitted to the rack 6 for directing the road wheels.
FIG. 8 is a block diagram showing the conventional structure of the control unit of the electric power steering device. The control unit 9 includes a motor controller 91, a clutch controller 92 and a motor current detector 93. The motor controller 91 calculates the motor current command level I.sub.M on the basis of the steering torque T and the vehicle speed V, and drives the electric motor 13 in accordance with the motor current command level I.sub.M. The clutch controller 92 generates the clutch control signal C in response to the vehicle speed V, etc., and outputs it to the electromagnetic clutch 14. When the vehicle speed V reaches a predetermined level (e.g., 50 km/h) corresponding to a high vehicle speed condition, the clutch controller 92 switches the clutch control signal C from ON to OFF. The motor current detector 93 consisting of a grounded resistor detects the actual motor current level I.sub.M and inputs it to the motor controller 91.
In response to the actual motor current level I.sub.M, the motor controller 91 performs the feedback control of voltage supplied to the electric motor 13, such that the actual motor current level I.sub.M' flowing through the electric motor 13 agrees with the motor current command level I.sub.M. The clutch controller 92 switches the clutch control signal C to the OFF state under a high vehicle speed condition, or when a system failure is detected on the basis of the signals from various sensors during a low vehicle speed.
FIG. 9a shows the ON/OFF state of the clutch control signal C relative to the vehicle speed V. When the vehicle speed V is below the predetermined vehicle speed V.sub.0, the clutch control signal C is ON, wherein the clutch current of the electromagnetic clutch 14 is about 1 A and the electromagnetic clutch 14 transmits the assisting steering torque of the electric motor 13 to the steering system. When the vehicle speed V is above the predetermined vehicle speed V.sub.0 , the clutch control signal C is OFF, wherein the clutch current is zero and the electromagnetic clutch 14 is disengaged.
FIG. 9b shows the variation of the motor current command level I.sub.M relative to the vehicle speed V (plotted along the abscissa) and the steering torque T (the increasing direction is shown by the arrow). The level of the motor current command level I.sub.M depends upon both the vehicle speed V and the steering torque T. The three curves in FIG. 9b corresponds to three distinct values of the steering torque T increasing in the direction of the arrow. When the vehicle speed V is above the predetermined vehicle speed V.sub.0, the motor current command level I.sub.M is zero for all values of the steering torque T. When the vehicle speed V is below the predetermined vehicle speed V.sub.0, the motor current command level I.sub.M becomes the greater as the vehicle speed V decreases and as the steering torque T increases. Usually, the steering torque required to turn the road wheels of the vehicle is greater when the speed of the vehicle is smaller. Thus, the motor current command level I.sub.M is increased as the vehicle speed V decreases, to provide a greater assisting torque.
Next, the operation of the conventional electric power steering control device of FIGS. 7 and 8 is described by referring to FIGS. 9a and 9b.
During the low vehicle speed condition where the vehicle speed V is below the predetermined vehicle speed V.sub.0, the clutch controller 92 of the control unit 9 turns on the clutch control signal C to engage the electromagnetic clutch 14 such that the torque of the electric motor 13 is transmitted to the steering system. Further, the motor controller 91 generates the motor current command level I.sub.M on the basis of the vehicle speed V from the vehicle speed sensor 10 and the steering torque T from the steering torque sensor 3, and drives the electric motor 13 to generate a necessary assisting steering torque.
The required steering torque becomes greater as the vehicle speed V becomes smaller. Thus, the motor current command level I.sub.M increases as the vehicle speed decreases. Further, if a greater steering torque is necessary due to the road surface condition, etc., the motor current command level is adjusted accordingly to provide the required assisting steering torque. The steering torque which the driver of the vehicle is required to provide is thus maintained substantially constant.
When a system failure is detected during a low vehicle speed condition, the clutch control signal C is turned off to disengage the electromagnetic clutch 14. The electric motor 13 is thus disconnected from the steering system to ensure safety. For example, when the motor current command level I.sub.M or the actual motor current level I.sub.M' exhibits an abnormality, or when the steering wheel 1 is not operated for a prolonged period, an occurrence of a system failure is inferred, and the clutch control signal C is turned off even under a low vehicle speed condition.
On the other hand, during a high vehicle speed condition where the vehicle speed V is above the predetermined vehicle speed V.sub.0, the motor controller 91 reduces the motor current command level I.sub.M to zero, and the clutch controller 92 turns off the clutch control signal C to disengage the electromagnetic clutch 14 and to disconnect the electric motor 13 from the steering system. Under this condition, the electric motor 13 is not driven since no voltage is supplied thereto. Further, the steering operation is not disturbed by an occurrence of a system failure since the electromagnetic clutch 14 is disengaged. When the vehicle speed V falls below the predetermined vehicle speed V.sub.0, the clutch control signal C is again turned on to engage the electromagnetic clutch 14, and the motor current command level I.sub.M is generated in accordance with the vehicle speed V and the steering torque T.
Generally, when the control unit 9 fails, an unnecessary motor current command level I.sub.M is generated to drive the electric motor 13 erroneously. The steering wheel 1 is thus turned against the intention of the driver. On the other hand, when the electric motor 13 fails mechanically, the steering wheel 1 becomes incapable of rotation and the driver loses control of the vehicle. The dangers resulting from these failures may be avoided by the driver of the vehicle during a low vehicle speed condition. However, when the vehicle speed is high, these failures present extreme dangers upon the safety of the driver. Thus, the clutch control signal C is turned off to provide a fail-safe measure under a high vehicle speed condition where the vehicle speed V exceeds the predetermined vehicle speed V.sub.0.
Thus the above conventional electric power steering control device has the following disadvantage. Namely, during a low vehicle speed condition (V&lt;V.sub.0), the electromagnetic clutch 14 is always turned on, provided that no system failure occurs. On the other hand, during a high vehicle speed condition (V.gtoreq.V.sub.0), the electromagnetic clutch 14 is always turned off irrespective of an occurrence of a system failure. Thus, each time the vehicle speed V varies across the predetermined critical vehicle speed V.sub.0 , the electromagnetic clutch 14 is turned on and off, thereby generating a large operation noise. The driving comfort is thus reduced. In addition, the repeated ON/OFF operation of the electromagnetic clutch 14 reduces the life thereof. Furthermore, the ON/OFF operation of the electromagnetic clutch 14 gives the driver a certain shock through the steering wheel 1.